Aromatic diamines and polyimides based on 4,4&#39;-bis-(4-aminophenoxy)-2,2&#39; or 2,2&#39;,6,6&#39;-substituted biphenyl

ABSTRACT

This invention relates the novel diamines, the polyimide oligomers and the polyimides derived therefrom and to the method of preparing the diamines, oligomers and the polyimides. The thermoplastic polyimides derived from the aromatic diamines of this invention are characterized as having a high glass transition temperature, good mechanical properties and improved processability in the manufacture of adhesives, electronic and composite materials for use in the automotive and aerospace industry. The distinction of the novel aromatic diamines of this invention is the 2,2&#39;,6,6&#39;-substituted biphenyl radicals which exhibit noncoplanar conformation that enhances the solubility of the diamine as well as the processability of the polyimides, while retaining a realatively high glass transition temperature and improved mechanical properties at useful temperature ranges.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without payment of any royaltiesthereon or therefore.

PRIOR U.S. APPLICATION

This Application is a Division of application Ser. No. 09/012,173 filedJan. 23, 1998 now U.S. Pat. No. 5,939,521.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to novel aromatic diamines, the polyimideoligomers and the polyimides derived from said diamines and polyimideoligomers. More specifically, this invention relates the novel diamines,the oligomers and the polyimides derived therefrom and to the method ofpreparing the diamines, polyimide oligomers and the polyimides. Thepolyimides derived from the aromatic diamines of this invention arecharacterized as having high glass transition temperatures, goodmechanical properties and improved processability in the manufacture ofadhesives, electronics and composite materials. The distinction of thenovel aromatic diamines of this invention over the prior art resides inthe 2,2'- and 2,2', 6,6'-substituted biphenyl moieties which exhibitnoncoplanar conformation that enhances the solubility of the diamine aswell as the processability of the polyimides, while retaining arelatively high glass transition temperature and improved mechanicalproperties at useful temperature ranges.

The novel aromatic diamines and the polyimides derived therefrom inaccordance with this invention can be characterized by the followingformula and by the disclosure more specifically set forth herein.##STR1##

BACKGROUND OF DISCLOSURE

Polymeric materials having improved high-temperature characteristicshave been required to enhance the performance and to reduce the weightof various industrial materials in the fields of electronic devices,aeronautical equipment and machinery. The polyimides are known to havethe required mechanical strength, dimensional stability, flameretardance, low coefficient of thermal expansion, and electricalinsulation properties in addition to excellent high-temperatureresistance. Polyimides of high molecular weight, however, generally havea high softening point and are insoluble in most organic solvents.Therefore, many difficulties have been encountered in the use ofpolyimides.

While there are a number of prior art thermoplastic polyimides that haveimproved heat resistance and mechanical strength compared to the generalpurpose plastics, there is still a need for thermoplastic polyimideresins having improved heat-resistance, and good mechanicalcharacteristics.

For example, polyimides have been prepared from the following4,4'-bis(4-aminophenoxy)-biphenyl, ##STR2## This particular diamine isinsoluble in many of the common solvents such as ethanol, acetone orN,N-dimethylformamide (DMF), but is soluble only inN-methyl-2-pyrrolidinone (NMP).

Further, in the prior art, for example, U.S. Pat. No. 5,344,986, Orenet.al. discloses a composition and the process of making a4,4'-bis(4-amino-substituted phenoxy) diamine. U.S. Pat. No. 5,268,487Yang et.al discloses the use of 4,4'-bis(4-aminophenoxy)3,3'-dimethyl-biphenyl and4,4'-bis(4-aminophenoxy)-3,3',5,5'-tetramethyl-biphenyl for thepreparation of polyimides. Although U.S. Pat. Nos. 5,344,986 and5,268,487 disclose structures similar to the diamines of this invention,the 2,2'- or 2,2',6,6'-substituents in the diamine of this inventionimprove the glass transition temperature of the corresponding polyimidesand enhanced the solubility of the diamine monomer. Recently, polyimidescontaining 2,2'-substituted benzidine was found to possess excellentthermo-oxidative stability as well as enhanced processability asdescribed in U.S. Pat. Nos. 5,071,997; 5,487,918; and 5,322,924.

However, the glass transition temperature of the thermoset polyimide(IV) derived from 2,2'-bis[-4-(4-aminophenoxy)phenyl] propane (BAPP)exhibits a lower glass transition temperature (˜280° C.) than thepolyimides derived from the diamine of this invention [T_(g) =307° C.].Moreover, the thermoset polyimides derived from the diamines of thisinvention exhibited physical properties 30% higher than the propertiesof the polyimides derived from 2,2-bis[4-(4-aminophenoxy)phenyl]propane. The advantage of the diamine of this invention is the fact thatthe 2,2'- or 2,2',6,6'-substituted biphenyl moieties exhibit noncoplanarconformation, which enhances the solubility of the diamine as well asthe processability of the polyimides, while retaining a relatively highglass transition temperature and better mechanical properties at auseful temperature range. In addition, the monomeric diamines of thisinvention are soluble in most of the common organic solvent such asacetone, tetrahydrofuran, etc. The substituents in the diamine improvesthe solubility of the diamine in solvents in comparison to theunsubstituted diamines of the prior art.

SUMMARY OF THE INVENTION

This invention relates to novel substituted aromatic diamines, thepolyimide oligomers and polyimide resins derived from said aromaticdiamines. The polyimides are characterized as having high glasstransition temperatures (T_(g)), good mechanical strength and are easyto process in the manufacture of electronics, adhesives and compositesfor use in the automotive, space, aeronautical and building industries.

More specifically, this invention relates to aromatic diaminescharacterized by Formula I: ##STR3## and the use of said diamines in thepreparation of lower molecular weight polyimide oligomers (Formula III)and high molecular weight thermoplastic polyimides (Formula II)characterized by the following formulae: ##STR4##

In formulae I, II and III, A is a radical selected from the Groupconsisting of an alkyl radical of 1 to 4 carbons, --CF₃, aryl, halogen,--OR where R is an alkyl, aryl or substituted aryl radical of 1 to 8carbons, and --OCX₃ where X is halogen, and B is a radical selected fromthe Group consisting of hydrogen, an alkyl radical of 1 to 4 carbons,--CF₃, aryl, halogen, --OR where R is an alkyl, aryl, or substitutedaryl radical of 1 to 8 carbons, and --OCX₃ where X is halogen. Informulae II and III, Ar is a tetravalent aromatic group such as adianhydride. In formulae II and III, "n" is a whole number ranging from2 to 100 and in formula III, E is an unreactive aromatic chain blockersuch as aniline, phthalic anhydride and the like or a reactive unitwhich can undergo further crosslinking to form a network.

Accordingly, an object of this invention is to provide an aromaticdiamine for use in the preparation of thermoplastic polyimidescharacterized as having high glass transition temperatures and goodmechanical strengths for the preparation of various industrial products.

It is another object of this invention to provide aromatic diamines andpolyimides that are easy to process due to the improved solubility ofthese products in various organic solvents for the manufacture ofelectronics, adhesives, and particularly the manufacture of compositesuseful in the space and aeronautical industries.

It is a further object of this invention to provide the method ofpreparing high molecular weight polyimides from the novel substitutedaromatic diamines.

It is still a further object of this invention to provide high molecularweight polyimides useful in the preparation of polyimide resincomposites having improved mechanical properties.

These and other objects of the invention will become apparent from afurther and more detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to novel substituted aromatic diamines and theuse of these diamines in the preparation of polyimide oligomers and highmolecular weight thermoplastic polyimides. The distinction of thediamine of this invention over the prior art is due to the 2,2'-or2,2',6,6'-substituted biphenyl radicals in the diamine which exhibitnoncoplanar conformation, that enhances the solubility of the diamine aswell as the processability of the polyimides derived therefrom, whileretaining relatively high glass transition temperatures and bettermechanical properties.

An example of preparing the substituted aromatic diamines of thisinvention is the following: ##STR5##

More specifically, the synthesis of4,4'-bis(4-aminophenoxy)-2,-2'-dimethylbiphenyl is as follows: ##STR6##

EXAMPLE I Synthesis of 2,2'-dimethyl-4,4'-biphenol (2)

To a 1000 ml of flask, 2,2'-dimethylbenzidine dihydrochloride (53.4 g,10.15 mol) (1) was added as wet cakes, containing 26% moisture, asreceived along with 200 ml. of distilled water and 30 ml. ofconcentrated HCl. The resulting heterogeneous reaction mixture wasstirred at 0° C. Then a solution of sodium nitrite (22.77 g, 0.33 mol)in 40 ml. of water was added dropwise to the above solution over theperiod of 1 hour at 0° C. under nitrogen to form diazonium salts.Separately, a two phase solution containing 75 ml. of sulfuric acid in250 ml. of water and 125 ml of 1,2 dichloroethane in a 2000 ml.round-bottom flask was stirred vigorously into a one phase system at85-90° C. Then the diazonium solution was added dropwise to the abovetwo phase solution with very efficient stirring for 1-2 hours until nomore nitrogen was evolved due to the decomposition of diazonium slats.During the process, the diazonium salts were decomposed by aqueoussulfuric acid to form the biphenol and were immediately extracted to theorganic layer. The reaction mixture was cooled down, and the organiclayer was separated and dried over magnium sulfate. The solvent wasconcentrated to half of its original volume and then cooled in therefrigerator overnight to induce crystallization. The resulting solidswere collected and washed with 1,2-dichloroethane/hexane=15/85 to removethe dark color impurities to afford 19.66 g (62%) as the first crop.mp=106-108° C.

Synthesis of 4,4'-bis(4-nitrophenoxy)-2,2'-dimethylbiphenyl (3)

2,2'-Dimethylbiphenol (2) (42.8 g, 0.2 mol) was dissolved in 250 ml ofN,N-dimethylformaide (DMF), and then potassium carbonate (60.8 g, 0.44mol) and 4-fluoro-nitrobenzene (59.22 g, 0.42 mol) were added. Thereaction mixture was heated to reflux for 20 hours overnight. Thereaction mixture was filtered to remove potassium carbonate, then thesolution was concentrated to 1/3 of its original volume in a rotaryevaporator. Subsequently, water was added to the DMF solution toprecipitate out the product in quantitative yield, and then the productwas washed with ethanol to remove trace of unreacted4-fluoro-nitrobenzene. The crude product (mp 146-147° C.) is pure enoughfor next step.

Synthesis of 4,4'-bis(4-aminophenoxy)-2,2'-dimethylbiphenyl (4)

4,4'-Bis(4-nitrophenoxy)-2,2'-dimethylbiphenyl (80 g) was dissolved in350 ml of DMF and added carefully to a hydrogenation bottle containing 8gm of 5% Pt/C. The solution was subjected to hydrogenation at 100° C.for 3 hours. Then 100 ml of water and 3 g of decolorizing charcoal wereadded and heated for 5 min. The solution was then filtered through aCelite pad and 1000 ml of water was added. The reaction mixture became amixture of fluffy solid and a sticky resin and was stirred for 1 hour togive a tan colored solid. The solid was removed by filtration, crushedwith a mortar and pestel and then stirred with 1000 ml of water for 1hour. The solid was collected by filtration and dried to afford 59.2 g(85%) of the product mp=140° C.

The following is an example of preparing a polyimide of this invention.

EXAMPLE II

4,4'-Bis(4-aminophenoxy)-2,2'-dimethylbiphenyl (0.99 g, 2.5 mmol) and3,3',4,4'-benzophenonetetracarboxylic dianhydride (0.8 g, 2.5 mmol) weremixed with 10 g of dry N-methyl-2-pyrrolidinone(NMP), and the reactionwas stirred at room temperature overnight under nitrogen to obtain veryviscous poly(amic acid) solution. The viscous solution was heated toreflux at 200° C. for 2-3 hours. The resulting polyimide solution wasdiluted with additional dry NMP to the consistency of a maple syrup, andthen precipitated into ethanol to obtain colorless fibers.

More specifically, the aromatic diamines of formula I can be polymerizedwith effective amounts of at least one aromatic tetracarboxylic acid,the anhydrides or the esters of said tetracarboxylic acid to obtain thecorresponding polyimides. The process of preparing the thermoplastic,high molecular weight polyimides (n=2 to 100) of this invention isdescribed by the reaction of a dianhydride and the aromatic diamine offormula (I) as follows: ##STR7##

The Ar group indicates an aliphatic or aromatic radical having a valenceof four. ##STR8##

An alternative embodiment of this invention includes the preparation ofpolyimide copolymers containing the aromatic diamine of formula I. Forexample, a mixture of two or more different dianhydrides or twodifferent diamines can be used where at least one of them is the diamineof formula I. Specific examples of the preferred anydrides which may beemployed in this invention includes pyromellitic dianhydride,4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride),3,3',4,4'-benzophenonetetracarboxylic dianhydride,2,3,3',4'-benzophenonetetracarboxylic dianhydride,3,3',4,4'-biphenyltetracarboxylic dianhydride,2,3,3',4'-biphenyltetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride, bis(3,4-dicarboxyl)sulfonedianhydride, 4,4'-(p-phenylenedioxy)diphthalic anhydride,1,2,5,6-naphthlenetetracarboxylic dianhydride,2,3,6,7-naphthlenetetracarboxylic dianhydride,1,4,5,8-naphthalenetetracarboxylic dianhydride etc. and thecorresponding tetracarboxylic acids or esters.

In addition, the polyimide oligomers of this invention can be preparedwith an aromatic dianhydride, its diester or diacid reacted with thediamine of formula I and a chain stopping reactant capable of furthercrosslinking. These reactants can be mixed in the ratios needed toobtain the polyimide oligomers of formula III. The oligomers can undergofurther crosslinking through the reactive units associated with endcap Eas follows: ##STR9##

In the above reaction, E can be an unreactive chain blocker such asaniline or a phthalic ester or phthalic anhydride. E can also be areactive unit wherein E further crosslinks with linear oligomeric chainsto form a network IV. Specific examples of some preferred reactive unitsor reactive endcaps (E) are as follows: ##STR10##

The use of the aromatic diamine of formula I improves the processabilityof the resulting polyimides while retaining a relative high Tg, andthereby raises the useful temperature range. Furthermore, higher T_(g)generally translated into better mechanical properties over usefultemperature ranges.

Specific examples of the preferred aromatic diamines of this inventionfor purposes of preparing the thermoplastic polyimides of this inventioninclude: ##STR11##

One of the objects of this invention was to evaluate several resins for500-550° F. applications, see Table I, using a solvent assisted resintransfer molding (RTM) process. PMR-15, APDB-20 and AMB-21 polyimidecomposites were prepared from 3,3',4,4'-benzophenone tetracarboxylicacid, dimethyl ester (BTDE) and each respective diamine; namely,4,4'-methylene dianiline (MDA),4,4'-bis(4-aminophenoxy)-2,2'-dimethylbiphenyl (APDB) and2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), using Nadic acid ester(NE) as the endcap. Alternately, BAPA-16 composites were prepared frombisphenol-A tetracarboxylic acid, dimethyl ester, p-phenylenediamine(p-PDA) and nadic ester (NE). The glass transition temperature (T_(g)),the thermo-oxidative stability and the mechanical properties of thesepolyimide/carbon fiber (T650-35) composites were compared to PMR-15; seethe data in Table II.

Monomer solutions of the polyimides (Table I) were prepared from 50 wt %methanol or methanol-acetone. The prepreg tapes were made by brushapplication of monomer solution onto drum-wound T650-35 carbon fibers,and subsequently dried. The laminates were then fabricated from 12 pliesof unidirectional prepreg by a simulated autoclave preocess. The T_(g)of these polyimide composites were measured by dynamic mechanicalanalysis (DMA) on a Rheometrics RMS 800 and thermal mechanical analysis(TMA).

The DMA and TMA data (Table II) shows that APDB-20 composites displayhigher T_(g) than either AMB-21 or BAPA-16, because the2,2'-dimethylbiphenyl moiety is more rigid than the isopropylidene[--C(CH₃)₂ --] group present in AMB-21 and BAPA-16. Furthermore, the twomethyl substituents on APDB-20 forces the biphenyl rings into adopting anoncoplanr conformation, which enhances the solubility of the APDB-20diamine and the processability of the APDB-20 oligomers. Essentially,APDB-20 polyimide can be processed like AMB-21. The mechanicalproperties of PMR-15, APDB-20 and AMB-21 composites at 550° F. followthe trend of higher T_(g) 's leading to better mechanical properties inthe order of PMR-15>APDB-20>AMB-21. The isothermal aging study at 500°F.(288° C.) indicated that APDB-20 exhibited higher weight loss, butstill retained about 70% of mechanical properties compared to PMR-15.This phenomenon is attributed to the loss of methyl substituents onAPDB-20 due to thermo-oxidative degradation, however, the polymerbackbone apparently still remains intact. When testing at 500° F.(260°C.), APDB-20, AMB-21 and BAPA-16 all displayed comparable initialmechanical properties, except the flexural strength is slightly lower inBAPA-16.

    TABLE I       -     Re-           peating        Endcap Dimethyl Esters Diamine unit (n)       Molar Ratio 2 n n +      1 2       APDB-20      ##STR12##      ##STR13##      ##STR14##       AMB-21 NE BTDE      ##STR15##       PMR-15 NE BTDE      ##STR16##      2.087       BAPA-16 NE      ##STR17##      ##STR18##

                  TABLE II                                                        ______________________________________                                        T.sub.g 's of Polyimide Composites by DMA.sup.a and TMA.sup.b                               G' (onset).sup.c                                                                          Tan δ                                                                             TMA                                         Property ° C. ° C. ° C.                                Resin    NPC.sup.d                                                                              APC.sup.e                                                                            NPC    APC  NPC    APC                               ______________________________________                                        APDB-20  282      307    320    334  269    307                                 AMB-21 241 280 270 304 243 278                                                BAPA-16 277 281 306 308 252 273                                               PMR-15 345 348 375 376 320 348                                              ______________________________________                                         .sup.a DMA = Dynamical mechanical analysis at a heating rate of 5°     C./min by a Rheometric RMS 800, using a torsional rectangular geometry at     1 Hz and 0.05% tension.                                                       .sup.b TMA = Thermal mechanical analysis by expansion probe, with 5 g loa     and a heating rate of 10° C./min.                                      .sup.c G' = Onset decline of strorage modulus                                 .sup.d NPC = No postcure                                                      .sup.e APC = Air postcure at 600° F. (315° C.) for 16 h.   

In comparison, the APDB-20 polyimide, which contains 2,2'-dimethylbiphenyl moiety, exhibited higher T_(g) and mechanical properties thanAMB-21 or BAPA-16, that consisted of the flexible isopropylidene group.Although the ether linkages in these polyimides tend to improve theprocessability, they generally result in lower T_(g) and poorerthermo-oxidative stability, in comparison to PMR-15.

The polyimide and carbon fiber composites of this invention can be usedas a lightweight replacement for metallic components in the aerospacefield, due to their outstanding thermo-oxidative stability andmechanical properties. The polyimide matrices offer better propertyretention over epoxies or bismaleimides in high temperature environment.The polyimide composites are often fabricated using hand lay-uplaminates by vacuum bag autoclave or compression molding methods,instead of injection or resin transfer molding (RTM). Carbon fiberswhich can be used with polyimide include acrylic carbon fiber,rayon-based carbon fiber, lignin-based carbon fiber and pitch-basedcarbon fiber. The form of carbon fiber can be chopped strand, roving andwoven fabric. In order to apply the polyimide to the carbon fiber, thediamine monomer is dissolved in a solvent such as methanol, acetone,N,N-dimethylacetamide, or N-methyl-2-pyrrolidione. The amount of carbonfiber and polyimide resin are mixed to make the composition of thisinvention ranges from 10 to 70 parts by weight of the carbon fiber tofrom 90 to 30 parts by weight of the polyimide resin.

Additives that can be incorporated with the polyimide composition ofthis invention includes talc, calcium carbonate, mica, and otherfillers, glass fiber, ceramic fiber and other fibrous reinforcements.These additives can be used in amounts depending on the quality andperformance of the composition. The polyimide resin composition can beprocessed into desired articles by injection molding, extrusion forming,transfer molding, compression molding and other known processingmethods. The polyimide resin compositions of this invention haveexcellent mechanical strength at high temperatures and therefore can beused for mechanical parts which requires high mechanical strength athigh temperatures.

While this invention has been described by a number of specificexamples, it is obvious that there are other variations andmodifications which can be made without departing from the spirit andscope of the invention as particularly set forth in the appended claims.

The invention claimed:
 1. An aromatic diamine characterized by theformula: ##STR19## wherein A is a radical selected from the Groupconsisting of --CF₃, aryl, halogen, --OR where R is an alkyl, aryl orsubstituted aryl radical of 1 to 8 carbons, and --OCX₃ where X ishalogen, and B is a radical selected from the Group consisting ofhydrogen, an alkyl group of 1 to 4 carbons, --CF₃, aryl, halogen, --ORwhere R is an alkyl, aryl, or substituted aryl radical of 1 to 8carbons, and --OCX₃ where X is halogen.
 2. The aromatic diamine of claim1 wherein B is hydrogen.
 3. The aromatic diamine of claim 1 wherein B isan alkyl radical of 1 to 4 carbons.
 4. The aromatic diamine of claim 1wherein A is a CF₃ group.
 5. The aromatic diamine of claim 1 wherein Ais an aryl radical and B is hydrogen.
 6. The aromatic diamine of claim 1wherein A is CF₃ and B is CF₃.
 7. The aromatic diamine of claim 2wherein A is an aryl radical and B is hydrogen.
 8. The aromatic diamineof claim 1 wherein A and B are aryl radicals.
 9. The aromatic diamine ofclaim 1 wherein A is an --OR radical where R is an alkyl group and B isan --OR radical where R is an aryl group.
 10. The aromatic diamine ofclaim 1 wherein A is an --OCX₃ radical where X is halogen and B is an--OCX₃ radical where X is halogen.
 11. The aromatic diamine of claim 10wherein X is chlorine.